"to inform someone of the important facts," usually with in, 1934, from clue (n.). Related: Clued; cluing. Earlier in now-obsolete sense of "follow or track by clues" (1660s). In nautical use, "to haul up (a sail) by means of the clue-lines," from clue (n.) in the "wound ball of yarn" sense.

1590s, spelling variant of clew "a ball of thread or yarn," in this sense with reference to the one Theseus used as a guide out of the Labyrinth. The purely figurative sense of "that which points the way" is from 1620s. As something which a bewildered person does not have, by 1948.